The hyperpolarization-activated cation channel called I/h is a slow inward current and permeable to both Na+ and K+. It has been found in a diverse group of cell types in various species and contributes significantly to the resting membrane potential, neuronal firing patterns, and the limitation of excessive hyperpolarization. Although well documented and important physiological functions are known for this current, many unanswered questions remain. The molecular components of these channels have been identified recently and belong to a closely related gene family called the HCN gene family. The major focus of this proposal is to understand the relationships between the identified HCN channels, possible additional subunits, and the native currents. The expression patterns of HCN channels determined by RNase protection assays and in situ hybridization will first be compared to the expression of the native current. The biophysical and pharmacological properties of homo/heteromeric HCN channels will be studied systematically using heterologous expression systems and compared to that of the native currents. The heteromerization among different HCN channels will also be investigated using coimmunopreciptation. The hypothesis that native channels requires an auxiliary subunit will be further explored. This study is essential to further investigate the mechanism generating channel diversity and to determine the contribution of this channel to physiological and pathological function.